The invention relates to a device for material processing by means of laser radiation, said device comprising a source of laser radiation emitting pulsed laser radiation for interaction with the material; optics focusing the pulsed processing laser radiation to a center of interaction in the material; a scanning unit shifting the positions of the center of interaction within the material, wherein each processing laser pulse interacts with the material in a zone surrounding the center of interaction assigned to said laser pulse so that material is separated in the zones of interaction; and a control unit which controls the scanning unit and the source of laser radiation such that a cut surface is produced in the material by sequential arrangement of zones of interaction.
The invention further relates to a method of material processing by means of laser radiation, wherein pulsed processing laser radiation is generated, focused for interaction to centers of interaction in the material, and the positions of the centers of interaction in the material are shifted, wherein each processing laser pulse interacts with the material in a zone surrounding the center of interaction assigned to said laser pulse and material is separated in the zones of interaction and a cut surface is produced in the material by sequential arrangement of zones of interaction.
The invention further relates to a device for material processing by means of laser radiation, said device comprising a source of laser radiation emitting pulsed laser radiation for interaction with the material; optics focusing the pulsed processing laser radiation along an optical axis to a center of interaction in the material; a scanning unit shifting the positions of the center of interaction within the material, wherein each processing laser pulse interacts with the material in a zone surrounding the center of interaction assigned to said laser pulse so that material is separated in the zones of interaction; and a control unit which controls the scanning unit and the source of laser radiation such that a cut surface is produced in the material by sequential arrangement of zones of interaction.
The invention still further relates to a method of material processing by means of laser radiation, wherein pulsed processing laser radiation is generated and focused for interaction to centers of interaction in the material along an optical axis, and the positions of the centers of interaction in the material are shifted, wherein each processing laser pulse interacts with the material in a zone surrounding the center of interaction assigned to said laser pulse, and material is separated in the zones of interaction, and a cut surface is produced in the material by sequential arrangement of zones of interaction.
These devices as well as corresponding methods of material processing are particularly suitable to produce curved cut surfaces within a transparent material. Curved cut surfaces are produced, for example, in laser-surgical methods and, in particular, in ophthalmic operations. In doing so, treatment laser radiation is focused into the tissue, i.e. below the surface of the tissue, to a center of interaction. Material layers in a surrounding zone of interaction are separated thereby. The zone usually corresponds to the focus spot. The laser pulse energy is usually selected such that an optical breakthrough in the tissue forms in the zone of interaction.
In the tissue, a plurality of processes initiated by the laser radiation pulse take place in a time sequence after an optical breakthrough. First, the optical breakthrough generates a plasma bubble in the material. Once such plasma bubble has formed, it grows due to expanding gas. Next, the gas generated in the plasma bubble is absorbed by the surrounding material and the bubble disappears again. However, this process takes very much longer than the forming of the bubble itself. If a plasma is generated at a material interface which may even be located within a material structure, material removal is effected from said interface. This is then referred to as photoablation. In case of a plasma bubble separating previously connected material layers, one usually speaks of photodisruption. For the sake of simplicity, all such processes are summarized here by the term “interaction”, i.e. this term includes not only the optical breakthrough, but also any other material-separating effects.
For high precision of a laser-surgical method, it is indispensable to ensure high localization of the effect of laser beams and to avoid, if possible, collateral damage to adjacent tissue. Therefore, it is common in the prior art to apply the laser radiation in pulsed form so that the threshold value for the energy density required to initiate an optical breakthrough is exceeded only in the individual pulses. In this respect, U.S. Pat. No. 5,984,916 clearly shows that the spatial extent of the zone of interaction substantially depends on the pulse duration only as long as a pulse duration of 2 ps is exceeded. For values of few 100 fs, the size of the zone of interaction is almost independent of the pulse duration. Thus, high focusing of the laser beam in combination with very short pulses, i.e. below 1 ps, allows the zone of interaction to be inserted in a material with pinpoint accuracy.
The use of such pulsed laser radiation has recently become established, in particular, for laser-surgical correction of visual deficiencies in opthalmology. Visual deficiencies of the eye are often due to the fact that the refractive properties of the cornea and of the lens do not cause optimal focusing on the retina. This type of pulsing is also the subject matter of the invention described herein.
The aforementioned U.S. Pat. No. 5,984,916 describes a method of producing a cut by suitably generating optical breakthroughs, thereby ultimately exerting a selective influence on the diffractive properties of the cornea. A multiplicity of optical breakthroughs are sequentially arranged such that the cut surface isolates a lens-shaped partial volume within the cornea of the eye. The lens-shaped partial volume separated from the remaining corneal tissue is then removed from the cornea via a laterally opening cut. The shape of the partial volume is selected such that upon removal the shape and, thus, the refractive properties of the cornea are changed so as to cause the desired correction of a visual deficiency. The cut surface required here is curved and circumscribes the partial volume, thus necessitating three-dimensional shifting of the focus. Therefore, two-dimensional deflection of the laser radiation is combined with simultaneous shifting of the focus in a third spatial direction. This is summarized here by the terms “scanning”, “shifting” or “deflecting”.
When composing the cut surface by sequential arrangement of optical breakthroughs in the material, an optical breakthrough is generated many times faster than the time it takes until a plasma generated thereby is absorbed by the tissue again. It is known from the publication of A. Heisterkamp et al., Der Opthalmologe, 2001, 98:623-628, that, after an optical breakthrough has been generated, a plasma bubble forms in the eye's cornea at the focal point where the optical breakthrough was generated, which plasma bubble can grow together with adjacent bubbles to form macrobubbles. The publication explains that the joining of still growing plasma bubbles reduces the quality of the cut. Therefore, said publication proposes a method wherein individual plasma bubbles are not generated immediately adjacent to each other. Instead, a gap is left in a spiral-shaped profile between sequentially generated optical breakthroughs, which gap is filled with optical breakthroughs and the resulting plasma bubbles in a second pass through the spiral. This is intended to prevent joining of adjacent plasma bubbles and to improve the quality of the cut.
In order to achieve good quality of the cut, the prior art thus uses defined sequences in which the optical breakthroughs are generated. This is intended to prevent joining of growing plasma bubbles. Since a cut is desired, of course, wherein as few bridges as possible connect the material or the tissue, respectively, the plasma bubbles generated ultimately have to grow together in any case to form a cut surface. Otherwise, the material connections would remain and the cut would be incomplete. In this connection, DE 102005039833A1 suggests to lower the laser pulse energy to below the breakthrough threshold and to emit into the tissue several laser pulses in an overlapping manner, directly following each other in time.
Therefore, it is an object of the invention to generate the good-quality cuts in the material without having to observe defined sequences when introducing laser pulses.
According to the invention, this object is achieved in a first variant by a device of the first-mentioned generic type, wherein the control unit controls the source of laser radiation and the scanning unit such that adjacent centers of interaction are located at a spatial distance a ≦10 μm from each other. In the first variant, the object is further achieved by a method of the first-mentioned generic type, wherein adjacent centers of interaction are located at a spatial distance a ≦10 μm.
In a second variant of the invention, the object is achieved by a device of the first-mentioned generic type, wherein for each center of interaction the fluence F of the pulses is below 5 J/cm2. In the second variant, the object is also achieved by a method of the first-mentioned generic type, wherein the zones of interaction are exposed to pulses whose fluence F is below 5 J/cm2.
In a third variant of the invention, the object is achieved by a device of the second-mentioned generic type, wherein the control unit controls the source of laser radiation and the scanning unit such that the cut surface comprises two portions located adjacent to each other along the optical axis, and at least partially illuminates them with laser pulses applied within a time interval t≦5 s. Also in the third variant the object is achieved by a method of the second-mentioned type, wherein the cut surface comprises two portions located adjacent to each other along the optical axis which are at least partially exposed to laser pulses applied within a time interval t≦5 s.
The invention is based on the finding that zones of interaction in the material influence each other. Thus, the effect of a laser beam pulse depends on the extent to which previous laser exposures already took place in the vicinity of the center of interaction. From this, the inventors concluded that the pulse energy required to generate an optical breakthrough or to cause material separation depends on the distance from the nearest center of interaction. All of the variants according to the invention take advantage of this finding.
The inventive minimization of the distance between centers of interaction, e.g. of the distance between the focus positions of adjacent optical breakthroughs, according to variant 1 allows the processing pulse energy to be decreased. The parameter describing the pulse energy is the fluence, i.e. the energy per area or the areal density of energy. Thus, the inventive variant 1 with a distance of less than 10 μm addresses an aspect of the finding attributable for the first time to the inventors.
Another aspect is that the fluence of the processing laser pulses is now significantly reduced. Thus, variant 2 relates to the same aspect as variant 1, although it does not prescribe an upper limit for the distance, but for the fluence.